Parasitic begging calls of nestmate-evictor common cuckoos stimulate more parental provisions by red-winged blackbirds than calls of nest-sharing brown-headed cowbirds

Abstract: Parasites often manipulate host behaviors to achieve their own selfish fitness goals. However, the efficiency with which parasitic begging calls solicit foster parental care has not yet been compared across different avian obligate host-brood parasite systems. For example, the begging calls of nestmate-evictor parasites are predicted to solicit sufficient levels of foster parental provisioning for the sole parasitic chick in the nest, whereas the calls of nest-sharing parasites combine together with the begging by host nestmates to solicit provisions for the whole brood. We studied the red-winged blackbird (Agelaius phoeniceus), a host of the brood parasitic brown-headed cowbird (Molothrus ater), to test the effects of begging call playbacks of nestmate-evictor common cuckoos (Cuculus canorus) or nestmate-sharing cowbirds upon parental feeding patterns. As predicted, cuckoo calls elicited more parental feeding trips and the amount of food delivered than cowbird calls. Food provisioning to nestlings while hearing cuckoo begging calls was similar to that while hearing redwing calls, and both were higher relative to a control silence treatment, which suggested that the begging call acoustics of the cuckoo served as an efficient stimulus for parental care in this cuckoo-naïve host. The study also confirms that cowbird calls are not supernormal stimuli for parental provisions in this host species, as redwing begging calls themselves were similarly effective stimulants for conspecific parental provisioning behaviors. Future research should examine the acoustical bases and perceptual biases present in the cuckoo’s begging calls to efficiently solicit parental provisioning in both naïve and coevolved hosts. Significance statement: Nestlings of different avian brood parasites use distinct strategies to survive and solicit parental provisions by heterospecific foster parents: some of them evict or otherwise kill host nestmate to grow up alone in the nest (e.g., common cuckoos), whereas others typically share the nest with host chicks (e.g., brown-headed cowbirds). However, the efficiency to solicit parental care through begging calls by brood parasitic young with respect to their nestmate-evictor versus nest-sharing strategies has not yet been evaluated. We focused on a cuckoo-naïve and cowbird-exposed host species, the red-winged blackbird, to compare the effects of begging call playbacks of parasites and conspecifics, relative to silence controls. We found that the begging calls of cuckoos, rather than the cowbirds, elicited more parental feeding in this cuckoo-naïve host. This implied that it was the nestmate-evictor strategy, rather than host familiarity, that improved the efficiency of the parasitic begging calls. Our results provide the first experimental data on the efficiency of begging calls between cuckoos and cowbirds within the same host species, and it contributes to our understanding of the divergent ontogenetic strategies of evolutionarily distinct avian brood parasites.